The present invention relates to an internal drum laser image setter, i.e. an apparatus for exposing a film or plate covered with a light-sensitive material, the film or plate being supported to that it constitutes part of a circular cylinder ("drum"), to a laser light beam directed substantially from the center of the cylinder to the film or plate.
Internal drum laser image setters (also called internal laser image drum plotters) are used for high accuracy production of images comprising graphics, maps, and text. Examples of known internal drum laser image setters or plotters are disclosed in U.S. Pat. Nos. 4,853,709, 4,595,957 and 3,958,250, and in EP Patent Application No. 0 127 136. An example of a known, so-called external drum laser image setter is disclosed in U.S. Pat. No. 4,122,496.
An increasingly important use of internal drum laser image setters is for production of films for producing high-quality offset printing masks or plates, or for direct production of offset plates, i.e. so-called "prepress" work.
One type of internal drum laser image setter which has become widely accepted in the graphic industry as an efficient and accurate apparatus comprises a range of laser image setters of the types 5100 and 7100 from the company Purup Electronics A/S, Lystrup, Denmark. In these apparatuses, an assembly, a so-called "spinner assembly" comprising a rotating prismatic, light-reflecting element from which the laser light beam is directed to light-sensitive material arranged in the interior of the drum, is moved along a central axis of the drum of the apparatus. The laser light beam received by the prismatic element is output from a laser arranged at one end of the drum. The rotatable light reflecting element is rotated by means of a motor which constitutes a component of the spinner assembly.
The apparatus of the present invention is based on a construction principle which results in an even higher absolute accuracy than that obtainable in the known apparatuses and which provides a number of important advances and advantages which will be apparent from the following. The increased absolute accuracy is of importance, e.g., in four-colour print work, where four films produced in the apparatus are to be aligned in the preparations for the printing process.
The apparatus of the invention for exposing a film or plate comprising a light-sensitive material to a light beam comprises
an apparatus housing having an elongated cavity defining an inner support for supporting the film or plate in such a manner that the supported film or plate constitutes at least a part of a substantially circularly-cylindrical surface defining a central axis, PA1 laser means for generating the light beam, PA1 a light-directing assembly comprising a light-directing assembly housing, a laser light-emitter emitting laser light generated by the laser means, a rotatable optically reflecting element and an element-rotating motor, the laser light-emitter being constituted by the second end of an optical fibre means having a first light-receiving end and a second light-emitting end, the first light-receiving end being arranged in juxtaposition to the laser means for receiving the light beam therefrom, and the second light-emitting end being arranged in and supported by the light-directing assembly housing and being in juxtaposition to the rotatable optically reflecting element at a substantially fixed distance therefrom for emitting the light beam to the rotatable optically reflecting element, the rotatable optically reflecting element being arranged relative to the second light-emitting end of the optical fiber so as to direct the light beam emitted to the optically reflecting element to the light-sensitive film or plate, the element-rotating motor being supported by the light-directing assembly housing and having a rotatable output shaft, the optically reflecting element being connected to the output shaft so as to be rotatably driven thereby, and the light-directing assembly housing being supported by the apparatus housing and being movable relative thereto along the central axis, PA1 motion means for moving the light-directing assembly relative to the apparatus housing along the central axis, and PA1 central control means for controlling the laser means for switching the laser beam on and off, for controlling the operation of the element-rotating motor, and for controlling the operation of the motion means so as to expose a predetermined area of the photographic film by switching on the laser beam while the light-directing assembly is in a predetermined position relative to the apparatus housing and while the rotatable optically reflecting element is in a predetermined rotational position.
In the apparatus of the invention, the quality of the light beam, and thereby the quality and precision of the spot generation, may be considerably improved because of the use, according to the invention, of the optical fiber means. Thus, when transmitting the light beam through an optical fiber, irregularities in the laser light, especially ellipticity of the beam profile, will tend to be corrected during the passage through the fiber so as to result in a beam of light with a circular beam profile; in particular when the fibers are single-mode fibers with respect to the wavelength of the laser light, a circular Gaussian beam with known properties will be generated. Thus, in a preferred embodiment of the invention, the optical fiber means is a single-mode optical fiber with respect to the wavelength of the laser light, preferably a fiber with a glass core, and is of sufficient length to ensure substantial conversion of the light beam emitted from the laser into a circular Gaussian light beam with known properties.
A particularly advantageous feature of the apparatus of the invention is that even though the laser light emitter, which is positioned within the apparatus housing, is supported by the light-directing assembly housing and thus moves together with the optically reflecting element and is arranged in juxtaposition thereto at a substantially fixed distance therefrom, the actual source of the laser light, i.e. the laser of the laser means itself--which will normally generate very considerable heat which can adversely affect, for example, the light-sensitive material and/or the dimensional stability of components such as the light-directing assembly within the apparatus housing--may be located at a position relatively remote from heat-sensitive components within the apparatus housing, and will preferably be thermally insulated therefrom. In preferred embodiments of the apparatus, the laser means is arranged outside the apparatus housing proper. However, the laser means may also be arranged in and supported by the light-directing assembly housing, and thus, travel therewith. In this case the fiber should preferably be coiled up and arranged at the light-directing assembly and the laser should preferably be a laser having a relatively low heat production, such as a diode laser.
In previously known apparatuses in which the laser light emitter is arranged at a fixed position and the optically reflecting element moves relative to the laser light emitter, this movement of the light-emitter relative to the reflecting element may give rise to minor inaccuracy due to a incompletely accurate and insufficiently frictionless motion of the spinner assembly along the central axis of the internal drum. That these initially minor inaccuracies may become important is apparent, e.g., from the fact that in a large internal drum laser image setter which is capable of exposing a photographic film measuring 25".times.25", the distance from the light beam generating laser to the spinner assembly varies from a few inches to more than 25 inches.
An important advantage of the mechanical principle of the present invention is that improved accuracy is obtained making use of a construction which is simpler than that in the known apparatuses of lower accuracy. The attainment of adequate accuracy in a system comprising a fixed light-emitter (laser) on the one hand and a movable light-directing assembly on the other hand requires very precise adjustment of the whole apparatus, i.e. of the laser relative to the system comprising the apparatus housing and the moving light-directing assembly. In contrast, the constructional principles underlying the present invention make it possible to carry out fine adjustment of the apparatus housing per se, while the complete light-directing assembly comprising the laser light-emitter and the rotatable optically reflecting element can be adjusted as a small, separate element.
None of the patent documents mentioned above disclose or infer the possibility of using optical fibers to direct the laser light to desired locations within the apparatus, and neither do they disclose or infer any advantages associated therewith.
In preferred embodiments of an apparatus of the invention, the light beam generated by the laser means arranged outside or inside the apparatus housing is directed to the rotatable optically reflecting element of the light-directing assembly via the optical fiber means, thereby improving (vide supra) the quality of the beam and reducing the distance between the light-emitter and the rotatable optically reflecting element to a smaller, substantially fixed distance which is independent of the actual position of the light-directing assembly relative to the supporting apparatus housing.
Whereas previously known internal drum laser image setters appear to be limited to a single colour exposure from the single laser arranged at the end of the drum, the above-described construction using optical fibers makes it possible to use a laser means which comprises a plurality of lasers, the optical fiber means comprising a plurality of optical fibers or possibly fiber bundles, individual fibers or fiber bundles being connected to individual lasers. This makes it possible to adapt the apparatus of the invention to produce multicolour light exposure, such as exposure by light generated by means of a red, a green and a blue light source and together constituting a RGB (Red, Green and Blue) colour system. One important use of such an embodiment is the production of so-called proof prints, i.e. prints made on a photographic colour film which is exposed to multicolour light.
While temperature control is, of course, important with respect to the accuracy, control of the humidity is at least as important because of the pronounced tendency of most light-sensitive materials, such as photographic film, to undergo dimensional changes with changes in humidity. Thus, according to a very preferred embodiment of the invention, a domain of the apparatus comprising the apparatus housing and the light-directing assembly is encapsulated, preferably with a thermally insulating material, and the encapsulated domain is air-conditioned by means of a controllable air conditioning unit to permit control of temperature and humidity conditions within the domain.
The laser of the laser means of the apparatus of the invention may suitably be selected from argon ion lasers, HeNe lasers, HeCd lasers, frequency-doubled Nd:YAG lasers, and diode lasers, including frequency-doubled diode lasers.
The apparatus according to the present invention may comprise any appropriate rotatable optically reflecting element such as a rotating or rotatable mirror, a rotatable lens, a rotatable prism or the like or a combination thereof. In accordance with the presently preferred embodiment of the apparatus according to the present invention, the rotatable optically reflecting element is constituted by a rotatable optical prismatic element which is rigidly connected to the output shaft of the element motor and which is rotatable in a rotational motion defining a rotational axis which coincides with the central axis. In this preferred embodiment of the apparatus according to the present invention, the light beam emitted from the light-emitter is directed to the rotatable optical prismatic element along the central axis defined by the cylindrical supporting wall constituting the internal drum wall of the apparatus.
In the preferred embodiment of the apparatus the output shaft of the element-rotating motor is rotatably journaled in the light-directing assembly by means of air guidings, i.e. journals or bearings based on an air film, so as to reduce vibrations in the light-directing assembly.
Although the light-emitter, which in the presently preferred embodiment of the apparatus is constituted by the light-emitting second end of the optical fiber means, may be arranged so as to emit the light beam directly to the rotatable optically reflecting element, an iris means and a collimator means are preferably interposed between the light-emitter and the rotatable optical prismatic element and are supported by the light-directing assembly housing so as to render it possible to accurately control the transmission of the light beam from the light-emitting second end of the optical fiber means (constituting the light-emitter) to the rotatable optically reflecting element which reflects the light beam towards the photographic film supported by the inner support surface of the apparatus housing. Thus, the light beam may be accurately positioned, narrowed, spread and/or focused relative to the rotatable optically reflecting element.
Although the central control means of the apparatus may autonomously control the operation of the motion means and of the element-rotating motor of the light-directing assembly, the apparatus according to the present invention preferably further comprises an encoder means connected to the central control means and generating an encoder signal representing the rotational position of the rotatable optically reflecting element relative to the central axis. This encoder signal is suitably used by the central control means to control the motion means in accordance with the encoder signal. While a commercially available encoder means with an encoder disc mounted on a separate shaft connectable to the shaft on which the optically reflecting element is mounted would be useful in the apparatus of the invention, it has been found that an even higher degree of accuracy is obtained when the encoder disc is fixed directly to the same shaft as that to which the rotatable optically reflecting element is fixed, thereby eliminating small deviations deriving from the coupling of two shafts.
It is preferred that both the element-rotating motor and the motion means are operated under the continuous control of the central control means, the motion means preferably being operated in response to the encoder signal. The motion means may be a linear motor, a step motor, an electric AC or DC motor, a single- or multiphase motor or the like and/or hydraulically or pneumatically driven means and/or mechanical coupling means, such as shafts, gear boxes, toothed rods, toothed wheels, piezo worms, etc. or a combination thereof. In a presently preferred embodiment of the apparatus according to the present invention, the motion means comprises a threaded shaft, preferably a multi-threaded shaft, and a drive motor, such as a step motor, connected to the threaded shaft for causing the threaded shaft to rotate; in this case the apparatus also has a part, such as a threaded bore or a nut, meshing with the threaded shaft, preferably a recirculating planetary roller meshing with the threaded shaft. The drive motor may be supported by the apparatus housing or alternatively by the light-directing assembly housing, whichever is appropriate.
As the threaded shaft is caused to rotate by the drive motor, the meshing of the threaded bore or nut and the threaded shaft causes the light-directing assembly to move. As mentioned above, the drive motor is preferably a step motor, in particular a step motor which is adequately dimensioned to ensure that it will substantially always respond completely to the control by the central control means, thus rendering any sensing/feedback of the operation of the motion means superfluous.
The light-directing assembly housing may be journalled and supported by the apparatus housing in any appropriate manner by means of journalling means such as rollers, shafts, bearings or the like.
It is preferred, however, that the light-directing assembly housing is supported by the apparatus housing through a carriage which is supported by the apparatus housing through guiding means which permit the carriage to move parallel to the central axis.
In a preferred embodiment, the guiding means comprise two bearing elements extending longitudinally parallel to the central axis, one of the bearing elements supporting the carriage in a two-plane manner, the other bearing element supporting the carriage in a one-plane manner, the two bearing elements together thus supporting the carriage in a statically determined manner. (The term "statically determined manner" is used here in spite of the fact that the linear character of the bearing elements, as contrasted to three supporting points, does not permit a theoretically ideal static determination).
In a preferred embodiment, the carriage is pulled downwards towards the apparatus housing by magnetic attraction forces generated by magnets arranged longitudinally in the carriage or in the housing, so as to assist the gravity force in securing the carriage to the apparatus housing and to preload the carriage against the apparatus housing to secure permanent contact between the carriage and the apparatus housing through the bearing elements, maintaining the support of the carriage in a statically determined manner. This preferred embodiment ensures a high stability and precision and counteracts any tendency for the carriage to move in any direction transverse to the longitudinal direction parallel to the central axis.
The magnets are preferably arranged longitudinally in the carriage in such a manner that the concentration of magnetic forces is higher along a part of the carriage where the light-directing assembly is arranged so as to compensate for or counteract an added moment along that part.
The bearing means may be selected from ball or roller bearing assemblies, sliding guides, and aerostatic bearing assemblies. The bearing means are preferably ball bearing assemblies for providing a low friction and low vibration journalling of the entire light-directing assembly, and thus for providing a vibrationless and extremely accurate linear motion of the light-directing assembly and consequently of the rotatable optically reflecting element along the central axis of the apparatus housing.
The apparatus housing is preferably made from a material which provides high dimensional stability. One preferred material is cast iron. Other materials contemplated are polymer concrete or ceramics.
As indicated above, the underlying concept of the present invention renders it possible to provide a proof printer operating in accordance with the internal drum laser image concept, by providing a laser means comprising three lasers. In this case, the three lasers comprise a red laser, a green laser and a blue laser, and the optical fiber means comprises three separate optical fibers connected to the red laser, the green laser and the blue laser, respectively.
The red laser is suitably constituted by a red HeNe laser generating laser light of wavelength about 633 nm, or a red diode laser generating laser light of wavelength about 635 nm or of about 670 nm; the green laser is suitably constituted by a green HeNe laser generating laser light of wavelength about 543.5 nm, a green frequency-doubled Nd:YAG laser generating laser light of wavelength about 532 nm, or a green argon ion laser generating laser light of wavelength about 514.5 nm; and the blue laser is suitably constituted by a blue argon ion laser generating laser light of wavelength about 488 nm, a blue HeCd laser generating laser light of wavelength about 441 nm, or a frequency doubled diode laser generating laser light of wavelength about 420 nm.
Alternatively, for providing a single colour photographic exposure of a photographic film, e.g. in a YMCB (yellow magenta, cyan and black colour system), a single infra red (IR) laser, preferably a diode laser, or a single argon ion laser may be provided. The argon ion laser is the more expensive, but the photographic film used therefor is cheaper than the film used for the IR laser, and the argon ion laser provides the higher quality, i.e. a better defined light spot.
With an IR diode laser, the accurate switching on and off of the laser beam during the exposure process can be performed by simply activating and deactivating the laser. In order to render it possible to accurately switch the light beam emitter from the laser light beam on and off the laser means of the apparatus according to the present invention preferably comprises acousto-optical modulator means controlled by the central control means, such acousto-optical modulator means functioning by modulating the refractive index of a prism through which the laser beam passes, thus directing the laser beam in and out of the light path of the apparatus, respectively.